Field of the Invention
The invention relates to a method for manufacturing a thick turbine engine part, by using a method with injection of resin under pressure, such as an RTM (for Resin Transfer Molding) method.
In particular, the invention advantageously finds application for the manufacturing of rocket engine or aircraft engine parts, more particularly for making thick parts in a composite material with fiber reinforcements and a resin matrix, notably composite blades of turbine engine compressors.
Description of the Related Art
The RTM method is a method currently used in the manufacturing of aeronautical parts. It consists in making a preform in fibers, generally woven fibers, which is set into place in a closed mold where it receives by low pressure injection, a thermosetting resin with relatively low viscosity. The main steps of the method are placing the fiber preform in the mold, filling the mold by injection, polymerizing the resin and removing the obtained part from the mold.
For example, in the case of fan blades for turbine engines, a yarn or fiber structure is woven in order to produce a preform which will be used as a structural reinforcement for the parts. As an example, reference may be made to the methods for making preforms in woven yarns or fibers described in documents EP 1 526 285, FR 2 861 143 or FR 2 892 339, in the name of the applicant.
The three-dimensional preform is then placed in a mold inside an installation for injecting resin under pressure. Such an installation notably comprises an enclosure forming an autoclave inside which the mold containing the preform is positioned, a resin injection system connected to the mold via an injection channel, means, if necessary, for pressuring the resin composition and a vacuum source connected to the mold via a partial vacuum channel. In this way, a resin composition may be introduced under pressure at one end of the mold, for example at the base of the latter, and may advance towards another end of the mold, for example upwards, under the action of the partial vacuum established in the mold possibly completed with overpressure upon the arrival of the resin composition.
After injection of the resin, a polymerization step in the mold is carried out by applying given temperature and pressure profiles to the mold. The thereby obtained part is then cooled and then removed from the mold.
However, the step for removal from the mold recurrently has difficulties in extracting the molded part, which increase in the case of parts having large thicknesses such as the fans of turbine engines, in the area of their foot. In particular, due to the variation of the expansion between the mold and the blade during the cooling of the blade, the blade may remain blocked in the mold. This blocking is further reinforced by the presence of the injection channel which crosses the mold and supplies the preform with resin, directly from the foot of the blade (generally in its centre). After solidification, the polymerized resin present in this blocked channel actually blocks the part and should therefore be broken so as to be able to extract it from the mold.
This operation is generally carried out by means of a spatula with a sharp edge, which, in addition to severing the injection channel, when it is slipped under the part, gives the possibility of forcing its extraction.
These extraction conditions nevertheless locally damage the part and the mold, and represent a non-negligible waste of time considering the number of parts which generally have to be made.
Therefore, the creation of a large draft in the mold in the area adjacent to the injection channel was therefore proposed, i.e. typically at the foot area of the blade, in order to simplify its extraction.
However, the draft creates a pure resin area on the part which should then be mechanically removed in order to make the blade geometrically conform and to give it the intended dimensions, and not to perturb the subsequent machining operations. Moreover, it was observed that the inclined plane of the draft promoted deformation of the three-dimensional preform during its placement in the mold. Finally, making the draft does not give the possibility of solving the problems related to polymerization of the resin in the injection channel.
In document FR 2 892 339, it was also proposed to place shims at the edge of the mold and to first remove them before injection, during the cooling in order to avoid inducing stresses and embrittlement of the blade. However, the shape of the shims has to be adapted to each location where it is inserted, and thus does not give the possibility of solving the problems related to polymerization of the resin composition in the injection channel, so that the making of the draft remains necessary.
Document WO 2007/054615, as for it, proposes a method for manufacturing a part in composite material by injection of resin under pressure of the RTM type, applied by means of a tool notably comprising a chamber for storing resin, a working chamber 10 adapted for receiving the preform, and a line for transferring the resin from the storage chamber to the working chamber. The tool may further comprise a bridge fitting adapted to create a passage channel for the resin in the transfer line. However, this document does not give the possibility of solving the problem of the invention, i.e. facilitating the removal of the part from the mold. The bridge fitting is actually only used for reducing the possible air passages between both chambers.
Finally, document FR 1 520 113 describes a method for manufacturing a belt in a composite material by injecting rubber under pressure into a tool comprising a female portion, a male portion and an intermediate portion, positioned between the male portion and the female portion. The intermediate portion is crossed by a channel into which the rubber is injected, and delimits the ring-shaped internal surface of the belt. Nevertheless, this method is difficult to transpose to the manufacturing of a thick part of the turbine engine blade type, insofar that the injected composition is rubber and not a resin and therefore does not require any polymerization step.